A number of electrical power supply circuits such as wall switch units for lighting fixtures are potentially hazardous to individuals (e.g., repairmen). They comprise an ON/OFF switch or other identified or implied OFF function which most users assume isolates the circuit from the power source when the switch is off. In other words, a user may assume during servicing and maintenance that there are no live parts on the load side of the power supply circuit while the power supply circuit is in the identified OFF mode.
Until recently, safety requirements under Underwriters Laboratories (UL) standard 773 for nonindustrial photoelectric switches for lighting control have not been as stringent as requirements for other electric control circuits in different environments, and most ON/OFF switches and OFF mode identifying functions have been in compliance with UL 773. New safety standards have been devised, however, under the newly proposed UL 773A standard which requires an air gap switch in these types of electrical circuits. The newly proposed UL 773A standard requires that a power supply circuit incorporate either an air gap switch, or a solid-state switching device which restricts leakage currents to 0.5 milliamperes or less back to the load.
U.S. Pat. No. 4,713,598 discloses a power supply circuit 36 which comprises a current transformer XFR to derive operating current, as shown in FIGS. 1A and 1B. The primary winding W1 of the transformer XFR is in series with a switching mechanism SW (e.g., a relay). When the switching mechanism SW is closed, current flows through the primary winding W1 and is induced in the secondary winding W2. Voltage across the secondary winding W2 provides operating power via a power supply 42 (i.e., diode CR1 and capacitor C1) for the control circuitry 44 (i.e., sensor S and amplifier AMP). When the switching mechanism SW is open, the voltage differential for deriving operating current is across the secondary winding W2 to operate a power supply 42.
One of the drawbacks of this design is possible noncompliance with the newly proposed UL 773A safety standard. When the relay SW is open, the device 36 is still electrically connected to the AC source via the capacitor C2 and the secondary winding W2. When analyzed with electronic test equipment, it can be found on some devices that a 2.5 milliamp current flows through the secondary winding W2 of the transformer XFR even though the switching mechanism SW is in the OFF or open position and the load (e.g., a lamp) is no longer energized by the power source. Further, the device 36 does not appear to comprise energy or memory storage means for interrupting the full line to load current path when the load has been opened prior to the device 36 being put in an OFF position by, for example, a slide switch (not shown) or other identified or implied OFF switch. Thus, if the switch SW is a latching relay, and the lamp has burned open, it appears that a repairman could be exposed to full AC line current (e.g., 15 amperes). This is because the power supply circuit in FIGS. 1A and 1B does not provide means for changing the state of the switch SW, that is, no identified or implied OFF switch is provided to either directly or indirectly open the current path to the load. The lamp, therefore, is actually powered on until the relay SW is opened, regardless of whether the slide switch is placed in the OFF position. In addition, current transformers also have a minimum load requirement. Thus, a need exists for a power supply circuit which complies with the newly proposed UL 773A standard.